Parking locks are always used when non-mechanical components are involved in the transmission of the driving force in motor vehicles, especially in automatic transmissions with a hydrodynamic torque converter. In this way, despite the non-mechanical connection between the drivetrain and the motor of the motor vehicle, the drive wheels can be locked and unintentional rolling away of the vehicle can be prevented. For this purpose, as a result of an actuation signal issued by the driver, the parking lock blocks the transmission output shaft connected in a rotationally fixed manner to the drive wheels, at least in that a locking pawl arranged in the automatic transmission is brought into engagement with the teeth of a parking lock gearwheel fixed on the transmission output shaft.
As the transfer device, between the parking lock and the actuating device located in the passenger compartment of the motor vehicle for the vehicle's transmission, in principle various mechanical, electro-hydraulic and electro-mechanical systems are known. To improve the design options as regards the free arrangement of the actuating device in the passenger compartment, and also to ensure acoustical decoupling of the actuating device from the drivetrain of the motor vehicle, in modern motor vehicles or automatic transmissions the shift-by-wire principle is adopted more and more often, particularly also because modern automatic transmissions are more and more often controlled completely by electronic means.
In such “shift-by-wire” transmissions the shifting commands are as a rule transmitted from the actuating device or selector lever of the automatic transmission exclusively by means of electric or electronic signals. With regard to the parking lock, this means that actuation of the parking lock as well takes place by electric or electronic means, sometimes even automatically, for example in the case of an Auto-P function, namely when the parking lock is engaged automatically when the ignition key is withdrawn or the motor vehicle is left.
This, however, entails the additional requirement that in the event of failure of the electronic system or failure of the motor vehicle's electrical voltage supply, it must be possible to disengage the parking lock again by means of an emergency release system, in order to be able to move the vehicle even in such eventualities. For example, this is necessary when the vehicle is in a workshop or if it has to be towed in the event of a breakdown.
For this purpose shift-by-wire actuating devices for automatic transmissions are known, in which an additional mechanical connection for emergency use is provided between the selector lever—or between an independent emergency release actuator and the transmission, for example a cable. In this way, even if the electrical system has failed completely, the parking lock—after appropriate actuation of the emergency release system—can be released (and closed again) mechanically by means of the cable, so that the vehicle can in any case be moved.
Such mechanical connections or cables for emergency actuation of the parking lock in the transmission are usually connected by a pivoting lever to a shifting shaft of the transmission, so that if there is a failure of the voltage supply or of the transmission control system the shifting shaft, normally only moved by the transmission hydraulics, can also be moved for emergency operation purposes by the cable and pivoting lever and the parking lock can thereby be deactivated manually.
In normal operation, however, the problem then arises that the shifting shaft in the transmission and the emergency release pivoting lever connected thereto are in each case moved at the same time by the transmission hydraulics during shifting operations of the automatic transmission, whereby the mechanical emergency release transfer device between the transmission and the emergency release actuator—for example the cable—would also be moved. But this is undesirable, since it results in annoying noise in the emergency release transfer device and can also lead to unnecessary wear of the transfer device.
From European Patent EP 1 111 275 A1 a driving gear selector device with emergency release for releasing the parking lock is known, in which the cable used for emergency release is only loosely connected to the pivoting lever of the transmission shifting shaft by means of an abutment nipple arranged on the cable, but can otherwise slide freely along a slot-shaped recess of the pivoting lever.
In this way, in case of emergency, force transmission only takes place from the cable to the pivoting lever, but not conversely from the pivoting lever to the cable when the pivoting lever is moved by the transmission hydraulics during normal operation of the motor vehicle.
However, this technical principle known from the prior art is first of all potentially susceptible to corrosion in the area of the open, loose connection between the end of the cable and the pivoting lever. Moreover, precisely because of the loose end of the cable undesired noise can again be produced, which can then be conducted via the cable itself into the vehicle's interior. Furthermore this known technical principle requires an additional spring device on the cable to keep it tight while the transmission hydraulics moves the pivoting lever back and forth. This additional spring device adds to the cost and must also again be protected against corrosion and dirt. Owing to the only loose connection between the cable end and the pivoting lever, this known emergency release system is also prone to maladjustments during assembly, for example if during assembly the free standing length of the cable is measured wrongly.